So when is the last time you saw a mud floor? Better yet, when was the last wall mud job you saw? A mud job is tile slang for a mortar work. Prior to the ‘60s most work done in the tile world was over mortar bed in what was known as a "thickset" method. Thinset was not patented till the 1950s with additional patents all the way into the early 1960s. The original patents were issued to Henry Rothberg Sr., founder of Laticrete International and Tile Council of America Labs. It took quite a few years for this new product to catch on but it was to become a pivotal point in the affordability of ceramic tile installations. To lend some real insight into how great the impact of this step on the evolutionary tile chain really was, you need only look at the sales of ceramic tile products. Shortly after World War II ceramic tile sales were pretty much confined to 4 ¼ and 6-inch wall tile, quarry tile, and mosaic tile in the 25 Million square foot range. I started in the trade around 1970. Tile products were still relatively the same though we began seeing some 8-by-8 floor tile, sales then had already reached the range of 250 Million square feet, a ten fold increase. Mortar work was still a very strong part of the installation business, probably around 50 percent or better. Now with proliferation of backer boards, membranes, and well over a hundred different thinsets, sales have reached 3.5 Billion square feet. Of this, nearly 1 Billion square feet are installed over membranes or backer boards in lieu of mortar beds. Mortar beds were also required to bond tile to a concrete slab, there was no other means of bonding tile.
Prior to the invention of thinset, it took a structure of substantial substance and a tile person with a very high skill level to perform acceptable installations of ceramic tile. They used methods that had remained unchanged for thousands of years, dating back the Pyramids and Roman Empire. Some of these installations still exist, one being found at the bottom of a lake in a barge formally the property of a Roman Emperor. Mortar beds remain recognized as a Cadillac method of installation and those capable of doing them, captains of the tile installation world. For someone who posses the knowledge and skill to execute these types of installations the rewards can be great. However, these types of installations have continued to become rare, concentrated for the most part in large metropolitan areas and certain geographic parts of the US. While hard to accurately estimate how much tile remains installed in this manner, a good guesstimate would be in the 10 percent range. In what appears to be a little known fact, the United States is in the minority when it comes to thinbed (thinset) installations. Most other countries in the world build their homes out of masonry products and use a mortar bed type installation, maybe as high as 90 percent. In most foreign languages there is no word for "Backer Board." I could digress into some of the CTEF international training event stories but suffice it to say in one session our courteous attendees let us spend the better part of an afternoon doing a backer board product presentation as part of an installation training program only to ask the next day what is it and where do you use it? This was 2 years ago! They found thinset equally confusing.
Today's wooden structures do not readily lend themselves to mortar bed installations. Thickset or mud installations can be quite heavy. One inch of mortar weighs approximately 12 pounds per square foot. Most of our current code compliant residential structures allow 10 pounds per square foot as the weight of the entire structure. Very few mortar beds are only an inch in thickness, most range in the 1 ¼-inch to 2-inch range plus the weight of the tile. This type of installation system would easily require a minimum of an additional 20 pounds of load carrying capacity be added to the structure. To use an example: if you were to have a 2-by-10 floor joist in a normal residence on 16-inch centers, to meet the L/360 criteria of building code and tile installation the maximum length could be 16 feet. If you were to "beef-up" the floor system to handle our example of a mortar bed, that length would now become 13'5". Older homes, those built prior to the 1970s roughly speaking tended to be what we would today call over-built. The reasons for this change is both in the materials available and desire to value engineer today's modern homes. The current home building market is all about dollars and cents as anybody in construction knows, though, there can be some exceptions.
So just exactly what is this mortar we keep referring to? There are several terms to describe mortar work for tile. The most common is deck mud or dry-pack for floors and fat mud for walls. The first reaction most people have when they see and feel the consistency of mortar used for floors, countertops and benches is "that looks way too dry!" Floor mortar, commonly called deck mud, is probably not what you might expect if you are used to seeing concrete, brick mortar or stucco. All these materials use Portland cement, aggregates and typically water to begin a chemical reaction. Most of these cement products use what is called water of convenience. That means that more water than is necessary is used to provide ease of placement such as emptying a concrete truck or building a brick wall. In tile work, for ease of placement, an optimum amount of water is used, no more than necessary to start that chemical reaction. That allows for the material to be easily moved around the substrate as need in a fashion similar to that of building a sand castle, just enough water to hold its shape. Deck mud is not poured but rather placed. Excessively wet deck mud cannot be compacted because the moisture in the mix acts like a lubricant and creates more space between the aggregate particles. The original design of mortar installations was to provide a substrate and means of bonding ceramic tile. Today deck (floor or dry-pack) mud is used to produce flat or sloped setting surfaces for ceramic or stone tiles. It is especially helpful to produce flat surfaces for very large unit tile. Sometimes the mud is used over a plywood subfloor; sometimes it is used to over concrete. Methods of installation are varied depending on what the desired in service conditions are. There are various methods for floating mortar beds, bonded mortar beds, roof decks, wet area applications and wall installations. Deck mud can be used for residential or even extreme service industrial installations depending on the level of compaction applied to the mud.
The best deck mud is the result of three things: the right combination or ratio of dry and wet ingredients, proper mixing and compaction. For deck mud, the sand should be clean, sharp and slightly damp. Rounded sand grains offer only a fraction of the compressive strength achieved with sharp sand. Think of trying to pack a bunch of round golf balls in a box as opposed to a bunch of diamonds with their flat surfaces. Flat surfaces on the sand offer much more compactness and greater strength as well as easier handling. The proper mix ratio is determined by the needs of the in service conditions and tile installation. In some instances a latex additive is used for mortar when very specific needs are required. Use of latex additives in mortar beds can produce a material much harder than most concrete. When mixing with plain water, depending on application a sand/cement ratio between 6/1 and 4/1 is recommended. Mixtures rich in cement are prone to cracking and excessive shrinkage. Not enough cement yields a soft deck mud with very little compressive strength. When using a latex additive, always use the ratio recommended by the latex manufacturer. There are many ways to mix mortar and none are right or wrong, as long at it is thoroughly and properly mixed. On large installations, transit trucks, gas and electric powered mixers and even roto-tillers have been used to mix both floor and wall mud. Regardless of the method used, mixing must be complete enough for all the ingredients to become homogenized. One type of mixer that does not work is a concrete mixer. Mortar mixing needs a blade or paddle type configuration.
Mortar work is not difficult to Master, just takes some basic knowledge of sand and cement and a lot of time to practice. Those who do learn tend to have a life long preference to do mud work. Given the shortage of mud mechanics (yes, you would be a technically knowledgeable person in a manual trade; that makes you a mechanic), wages tend to be higher than those who do thinset methods exclusively. The success stories of those attending our mortar shower courses and mortar floor and wall basics are numerous and in some instances amazing to me. If our readers would like to pursue more information on the specifics of mortar floors and walls let us know and we will run an article on both or either in the future.